1. Field of the Invention
The present invention relates to methods for treating clays such as kaolin for the purpose of brightening the clay and thus rendering it more valuable for use in industries such as the paper industry and the coatings industry and more particularly refers to methods of treating clay in which a clay slurry is conditioned with activators and conditioning chemicals and subsequently floated in an air flotation procedure.
2. Prior Art
In U.S. Pat. No. 3,979,282 it is recognized that ferrous iron is capable of acting as an activator when added as a water soluble salt in a clay conditioning and flotation procedure. There is no disclosure or suggestion, however, in this patent or in any other prior art of taking steps to prevent the oxidation of the added ferrous salt before or during the conditioning operation to the ferric condition. More specifically, the patent fails to disclose or suggest the use of a reducing agent prior to or during conditioning to ensure that the ferrous salt remains in the ferrous condition.
The importance of maintaining the ferrous salt in the ferrous condition was not previously recognized in the prior art. For example, U.S. Pat. Nos. 3,655,038 and 3,923,647 actually introduce hydrogen peroxide just prior to conditioning or flotation. Such procedure clearly results in the conversion of any ferrous compounds originally in the clay into the ferric compounds.
U.S. Pat. No. 3,072,255 discloses the sulfidization of a dispersed clay with H.sub.2 S or Na.sub.2 S followed by conditioning and floating the clay slip. The procedure of this patent could result in the formation of a ferrous sulfide which is water soluble and which decomposes in hot water and is not generally accepted as an activator useful in conditioning clay prior to flotation. After sulfidization, the patent teaches neutralization and conditioning with oleic acid followed by addition of an alkaline material and particulate material such as graphite, sulfur, charcoal and the like followed by additional conditioning. There is no provision in the patent for maintaining the ferrous condition of the iron during conditioning of the slip. In addition the process of this patent needs to add particulate matter in order to assist in the float, i.e. in order to assist in improving the brightness qualities of the clay. The shortcomings of this patent in achieving an adequate brightness are admitted in U.S. Pat. No. 3,353,668 which refers to the process as providing "clay which has been partially brightened" and categorizes the clay treated by the process of U.S. Pat. No. 3,072,255 in the same category as raw clay (col. 2, lines 69-col. 3, line 1).
Bell et al U.S. Pat. No. 4,186,027, while not referring to a flotation process, discloses the reduction of iron in clay to the ferrous form using sodium dithionite followed by filtering to separate the iron from the clay.